CHAPTER 7 • ORGAN BLOOD FLOW
171
unit tissue weight. Whereas blood flow in
many organs is closely coupled to tissue oxi-
dative metabolism, this is not the case for
the kidneys, in which the blood flow greatly
exceeds the need for oxygen delivery. The
very high blood flow results in a relatively low
extraction of oxygen from the blood (about
1 to 2 mL O2/mL blood) despite the fact that
renal oxygen consumption is high (~5 mL O2/
min per 100 g). The reason for renal blood
flow being so high is that the primary func-
tion of the kidneys is to filter blood and form
urine.
The kidney comprises three major
regions: the cortex (the outer layer that con-
tains glomeruli for filtration), the medulla
(the middle region that contains renal tubules
and capillaries involved in concentrating the
urine), and the hilum (the inner region where
the renal artery and vein, nerves, lymphatics,
and ureter enter or leave the kidney). Because
most of the filtering takes place within the
cortex, about 90% of the total renal blood
flow supplies the cortex, with the remainder
supplying the medullary regions.
RENAL VASCULAR ORGANIZATION
The vascular organization within the kidneys is
very different from most organs. The abdominal
aorta gives rise to renal arteries that distribute blood
flow to each kidney. The renal artery enters the
kidney at the hilum and gives off several branches
(interlobar arteries) that travel in the kidney
toward the cortex. Subsequent branches (arcu-
ate and interlobular arteries) then form afferent
arterioles, which supply blood to each glomerulus
(Fig. 7.16). As the afferent arteriole enters the glo-
merulus, it gives rise to a cluster of glomerular
capillaries, from which fluid is filtered into Bow-
man capsule and into the renal proximal tubule.
The glomerular capillaries then form an efferent
arteriole from which arise peritubular capillaries
that surround the renal tubules. Efferent arterioles
associated
with
juxtamedullary
nephrons
Bowman’s
Capsule
■ FIGURE 7.16 Renal microvascular anatomy. Small vessels derived from branches of the renal artery form
arcuate arteries and interlobular arteries, which then become afferent arterioles that supply blood to the
glomerulus. As the afferent arteriole enters the glomerulus, it gives rise to a cluster of glomerular capillaries,
from which fluid is filtered into Bowman capsule and into the renal proximal tubule. The glomerular capillar-
ies then form an efferent arteriole from which arise peritubular capillaries that surround the renal tubules.
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